Silver nitrate which is of very high purity has many important industrial applications. For example, it is required for manufacture of photographic materials, for certain catalytic uses, and for use in the pharmaceutical industry. In the process that is generally used for preparing silver nitrate, metallic silver is reacted with nitric acid. A complex set of chemical reactions occurs during this reaction. Thus, reaction products other than silver nitrate are formed, including several oxides of nitrogen, such as nitric oxide (NO), nitrite ion (NO.sub.2.sup.-), and nitrous acid (HNO.sub.2). In this specification, each of these products is called a "nitrite"; and collectively they are called "nitrites". Nitrites are impurities that can have a negative effect on silver nitrate for the above-stated uses. For example, it is known that nitrite present in silver nitrate adversely affects photographic properties of the silver nitrate in photographic applications. Accordingly, various methods have been tried to decrease the level of nitrite in silver nitrate. Prior workers have shown that this is not an easy task. The approaches taken have met with varying degrees of success.
It is known, for example, that the nitrite concentration in a batch of silver nitrate solution tends to decrease with time. It is also known that adding nitric acid to the solution can lower nitrite concentration. A disadvantage is that, with or without nitric acid, the reaction time required to achieve a significant decrease is lengthy; consequently, a high residence time of the silver nitrate solution in the batch reactor is required. The high residence time requires a large inventory of silver nitrate for a desired amount of low-nitrite silver nitrate product. Manufacturing costs are therefore high because of high silver inventory costs and the large processing facilities required.
Other methods to remove nitrite from nitrate solutions have also met with limited success. In one prior an approach, an effluent containing nitrate and nitrite is mixed with nitric acid to form a solution. The solution is contacted with oxygen or a gas containing oxygen, for example air or a mixture of air and nitrogen oxides. The solution is introduced to a reaction tower to which a partial vacuum is applied, resulting in the conversion of nitrous acid to nitric acid and the evolution of nitric oxide. A vacuum pump draws off the nitric oxide. A disadvantage of the process is that an unacceptably high level of nitrite remains after it is carried out.
In another prior art method of removing nitrite from solutions containing nitrate such as silver nitrate, oxygen is introduced to entrain nitrogen oxides for further use in the starting reaction forming the solution. The solution is then directed through a layer of the metal species such as granulated silver and through a lower layer comprising an inert material where it forms additional product and is collected. This approach has the above-enumerated disadvantages of the prior art, wherein costs are high and nitrite is not decreased to an acceptable level.
Still another prior an method involves removing nitrite from a batch solution containing a nitrate species and nitrite by controlling nitric acid concentration within a specified range while the solution is agitated thermally and/or a gas is introduced. A disadvantage of this approach is that the final nitrite concentration can be unacceptably high, and further process steps such as crystallization to further decrease nitrite concentration can be unacceptably slow as is the case for all prior art processes. Solution residence time for the process and residence time in the crystallizer are high. A high silver inventory can therefore be necessitated, resulting in high inventory costs and costly production facilities.
U.S. Pat. No. 1,039,325 discloses a process of making nitrates and nitric acid in which an effluent containing nitrate and nitrite is mixed with nitric acid to form a solution. The solution is contacted with oxygen, or a gas containing oxygen, for example air or a mixture of air and nitrogen oxides. The solution is introduced to a reaction tower to which a partial vacuum is applied, resulting in the conversion of nitrous acid to nitric acid and the evolution of nitric oxide. A vacuum pump draws off the nitric oxide.
U.S. Pat. No. 2,581,519 discloses a method of manufacturing metal nitrates such as silver nitrate. A solution of silver nitrate is directed from a main reactor into a tube above a column containing two layers of material, an upper layer of granulated silver and a lower layer of inert material. When the solution commences to flow into the tube, oxygen is passed up into the tube to entrain nitrogen oxides present in the tube for further use in the reaction. The solution then flows into the column where it forms additional silver nitrate after which it is collected as product. Oxides of nitrogen not reconverted to nitric acid pass through a conduit into an absorption tower where they are converted to nitric acid.
Japanese Kokai No. 52[11977]-60294 discloses a batch process for the removal of nitrous acid and nitrite from a silver nitrate solution containing nitrite. The process is carded out by controlling nitric acid concentration in the solution within a specified range while the solution is agitated thermally and/or a gas is introduced.
Perry's Chemical Engineer's Handbook, 6th Ed., Section 18, "Liquid-Gas Systems", Fair et al, Section 18, pp. 1-48, (later referred to as "Perry's") discloses systems such as packed columns for carrying out operations such as liquid-gas absorption and distillations.
U.S. Pat. No. 2,189,363 discloses a zig-zag tube apparatus for carrying out a process for the absorption of nitrous gases in water.
U.S. Pat. No. 4,419,333 discloses a process for removal of nitrous gases from gaseous mixtures by scrubbing with nitric acid followed by oxidation.
U.S. Pat. No. 4,925,643 discloses regenerating an aqueous liquid comprising nitric acid and nitrous acid or nitrogen oxides in a packed column. An oxygen-containing gas is passed into the liquid, converting the liquid substantially to nitric acid in the column.
WO 81/00397 discloses a process for reconstituting nitrogen oxide gases to nitric acid by contacting the gases in a packed column with nitric acid to thereby convert the gases to nitric acid.